In order to increase the degree of filling in an ordinary aspirating engine, it can be provided with a turbocharger in order that the energy available in the exhaust gases, which would otherwise go to waste, can be better utilized. The exhaust gases are conducted through a turbine part for rotation of a turbine wheel. An impeller, which is in turn arranged in a compressor part for supercharging the air which is supplied to the engine, is mounted on the same shaft as the turbine wheel. A limitation of this type of turbocharger is that it can normally deliver the desired charging pressure only within a narrow speed and load range.
One type of turbocharger which solves this problem is known as a VGT (Variable Geometry Turbine) turbocharger, that is a turbine with adjustable geometry. With such a turbocharger, it is possible to widen this speed and load range by virtue of the geometry of the turbine part being adaptable for both small and large exhaust gas flows. A variant of the VGT, the Variable Nozzle Turbine (VNT) has continuously adjustable guide elements. With the aid of these, the exhaust gases can be directed towards the turbine wheel at different angles as a function of the prevailing exhaust gas flows. Broader turbine matching is thus obtained, and the desired charging pressure can be achieved for a wider speed and load range.
However, a high degree of turbine matching involves a compromise between on the one hand utilizing the small exhaust gas flows which are available at part load and low speeds and on the other hand handling the considerably larger exhaust gas flows which result at full load and high speeds.
For a VNT with adjustable guide elements, this means that the guide elements can be positioned so that the rate through the turbine is maximized for low exhaust gas flows. With this positioning of the guide elements, however, the exhaust gases will strike the turbine wheel relatively tangentially, which is not entirely optimum as far as force transmission is concerned. The consequence is poor utilization of the exhaust gases and thus impaired turbine efficiency. The opposite case arises with very large exhaust gas flows as the turbine power has to be limited and the guide elements occupy a diametrally opposite position in this case. However, the exhaust gases will strike the turbine wheel essentially radially, which is not favorable either as far as force transmission is concerned and also results in impaired turbine efficiency.
There are on the whole two methods for regulating this type of turbine. The first method is based on controlling the turbine power exclusively by adjusting the position of the guide elements. This requires a relatively high degree of turbine matching and as a rule results in poor torque and response at low speeds because it has to be possible to cater for a large speed and load range.
The second method is based on the turbine power being controlled not only by the position of the guide elements but also with the aid of some type of bypass arrangement, what is known as a waste gate. This means that any exhaust gas surplus, at full load and high speeds, when the guide elements already occupy the position intended for maximum exhaust gas flows, is ventilated out via the bypass arrangement. However, this regulating method represents poor utilization of the energy available in the exhaust gases because the turbine works with impaired efficiency.